1. Field of the Invention
The present invention relates generally to turbine airfoils, and more specifically to a turbine vane with endwall cooling.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as an industrial gas turbine engine, a compressed air from a compressor is passed into a combustor and burned with a fuel to produce a hot gas flow of extreme temperature. The high temperature gas is then passed through a multiple stage turbine where the heat energy is converted into mechanical energy used to drive the compressor and, in the case of an IGT drive an electric generator.
The efficiency of the engine can be increased by passing a high temperature gas flow into the turbine. In the turbine, the first stage stator vanes and rotor blades are exposed to the highest gas temperature. It is these airfoils that limit the turbine inlet temperature (TIT). Other than the material properties of these airfoils, a higher temperature can be used if higher levels of airfoil cooling can be used. However, airfoil cooling is wasted compressed air that lowers the engine efficiency since the compressed air used for cooling typically is bled off from the compressor. It is generally an objective of the design engineer to maximize the amount of cooling capability while at the same time minimizing the amount of cooling air used.
Under-cooling of turbine airfoils, like the endwalls of the stator vanes, can lead to hot spots that cause oxidation or backflow of the hot gas into the internal cooling passages of the airfoils. Both of these problems can severely shorten the life of the turbine airfoil and require excessive down times for the engine. In the case of an IGT, this can be very expensive, since these engines normally have to operate for thousands of hours without stopping.
In the prior art, backside impingement in conjunction with multiple rows of film cooling is used to provide cooling of the endwalls of a high temperature first stage stator vane as seen in FIG. 1. Individual compartments are used on the backside of the endwall for a better control of cooling flow and pressure distribution. Film cooling holes 11 open onto the surface of the suction side airfoil and the inner diameter endwall as seen in FIG. 1. The impingement holes 12 and the separated impingement compartments are shown on the outer diameter endwall in FIG. 1. However, to fit impingement pressure across the impingement holes or post impingement cooling air pressure, each individual compartment still experiences a large main stream pressure to cooling air pressure variation. Also, each impingement compartment has to be designed with a post impingement pressure higher than the maximum main stream hot gas pressure in order to achieve a good back flow margin (BFM) so that the external hot gas will not flow into the cooling holes. Consequently, an overpressure is produced at the lower main stream hot gas pressure location. This over-pressure issue becomes more pronounced at the aft portion of the vane suction side where the endwall is exposed to the maximum main stream variation as well as the maximum ratio of cooling air to hot gas pressure ratio.
Metering down the cooling pressure through the impingement holes extensively in order to obtain the maximum film cooling on the endwall surface may result in a hot gas ingestion problem when some of the impingement holes become plugged by dirt or other debris. As a result of this large compartment cooling construction design, it is difficult to achieve a stream-wise and circumferential-wise cooling flow control for an endwall with a large external hot gas temperature and pressure variation. In addition, a single impingement cooling technique with large impingement cavity to cover a large endwall region is not the best method of utilizing cooling air. The resulting mal-distribution of cooling flow yields a low convection cooling effectiveness.